Saturns E ring stretches nearly 200,000 miles from its inside edge to its outer bound and is made up of microscopic particles of ice and dust. The ring is very faint, with the brightest part around Enceladus, which, along with some of Saturn's other moons, travels through the plane of debris. This observation caused some scientists to suspect that Enceladus was somehow supplying material for the ring.

"The particles are so small they should not last very long. You couldn't have the E ring just sit there for the age of the solar system. It had to be regenerated somehow" - John Spencer, astronomer from the Southwest Research Institute in Colorado.

Two possibilities for how Enceladus could supply material for the ring were that impacts from cosmic particles were blasting tiny bits of Enceladus off into space, or that geysers or water volcanoes on the surface of Enceladus spewed out clouds of ice and dust into the moon's atmosphere, and because the moon is so small and its gravity so weak, the ice and dust soon float off into space. The discovery of the hot spot on the moon's south pole is important, because it provides the engine to supply the ring with material.

Why the south pole is so active is still a mystery. One theory is that radioactive material left over from the moon's formation billions of years ago is acting as a heat source.Another theory is that a change in the moon's spin rate caused fractures to form on the moon's surface.

This image shows the warmest places in the south polar region of Saturn's moon Enceladus. The unexpected temperatures were discovered by Cassini's composite infrared spectrometer during a close flyby on July 14, 2005. The image shows how these temperatures correspond to the prominent, bluish fractures dubbed "tiger stripes," first imaged by Cassini's imaging science subsystem cameras. Working together the two teams were able to pinpoint the exact location of the warmest regions on Enceladus.

The composite infrared spectrometer instrument measured the infrared heat radiation from the surface at wavelengths between 9 and 16.5 microns within each of the 10 squares shown here. Each square is 6 kilometres across. The colour of each square, and the number shown above it, describe the composite infrared spectrometer's measurement of the approximate average temperature of the surface within that square. The warmest temperature squares, at 91 and 89 degrees Kelvin, are located over one of the "tiger stripe" fractures. They contrast sharply with the surrounding temperatures, which are in the range 74 to 81 degrees Kelvin. The detailed composite infrared spectrometer data suggest that small areas near the fracture are at substantially higher temperatures, well over 100 degrees Kelvin.

Such "warm" temperatures are unlikely to be due to heating of the surface by the feeble sunlight striking Enceladus' South Pole. They are a strong indication that internal heat is leaking out of Enceladus and warming the surface along these fractures. Evaporation of this relatively warm ice probably generates the cloud of water vapour detected above Enceladus' South Pole by several other Cassini instruments. Scientists are unsure how the internal heat reaches the surface. The process might involve liquid water, slushy brine, or soft but solid ice.

The imaging science subsystem image is an enhanced colour view with a pixel scale of 122 metres that was acquired at the same time as the composite infrared spectrometer data. It covers a region 125 kilometres across. The spacecraft's distance from Enceladus was 21,000 kilometres. The broad bluer fractures that can be seen running from the upper left to the lower right of the image are 1 to 2 kilometres wide and more than 100 kilometres long. The fractures are thought to be bluer than the surrounding surface because coarser-grained ice (which has a blue colour just as thick masses of ice, like glaciers and icebergs, do on Earth) has been exposed in the fractures. The colour image was constructed using an ultraviolet filter (centred at 338 nanometres) in the blue channel, a clear filter in the green channel, and an infrared filter (centred at 930 nanometres) in the red channel.

The ion and neutral mass spectrometer and the ultraviolet imaging spectrograph onboard the Cassini spacecraft found the atmosphere contains water vapour.The mass spectrometer found the water vapour comprises about 65% of the atmosphere, with molecular hydrogen at about 20%. The rest is mostly carbon dioxide and some combination of molecular nitrogen and carbon monoxide. The variation of water vapour density with altitude suggests the water vapour may come from a localized source comparable to a geothermal hot spot.The ultraviolet results also strongly suggest a local vapour cloud.

Expand This shows a composite infrared (heat) spectrometer image of the south pole of Saturn's moon Enceladus.The right hand panel shows a global temperature image made from measurements of Enceladus' heat radiation at wavelengths between 9 and 16.5 microns. Cassini made the observation from a distance of 84,000 kilometres on the approach to Enceladus, and the image shows details as small as 25 kilometres. Image credit: NASA/JPL/GSFC.

Another Cassini instrument, the composite infrared spectrometer, also showed the south pole is warmer than anticipated. Temperatures near the equator were found to to be 80 degrees Kelvin. The poles should be even colder. However, the instrument found the average south polar temperatures reached 85 Kelvin, much warmer (by 15 degrees Kelvin) than expected. Small areas of the pole, concentrated near the "tiger stripe" fractures, are even warmer: well over 110 Kelvin in some places.

It is thought that a portion of the polar region, including the "tiger stripe" fractures, are warmed by heat escaping from the interior. Evaporation of this warm ice at several locations within the region could explain the density of the water vapour cloud detected by other instruments. But how a 500-kilometre diameter moon can generate this much internal heat and why it is concentrated at the south pole is still a mystery.

The fact is that an atmosphere persists on this low-gravity world, instead of instantly escaping into space. This suggests the moon is geologically active enough to replenish the water vapour at a slow continuous rate.

The 14th July 2005 close flyby of Saturn’s icy moon Enceladus by the Cassini spacecraft confirms that the moon has a significant, extended and dynamic atmosphere. The flyby was Cassini’s lowest altitude flyby of any object to date, a mere 173 kilometres above the surface of Enceladus.

Flybys of Enceladus which took place earlier this year (on February 17th at an altitude of 1,167, and on March 9th, 500 kilometres above the surface) had shown deformation or bending of the magnetic field around the moon, revealing that Enceladus was acting as a large obstacle to the flow of plasma and magnetic field from Saturn by its extended asymmetric atmosphere.

The recent close flyby confirms and extends the observations from the two other flybys.The magnetic data also suggests that the atmosphere is not symmetric and may be arising from a comet-like jet from the southern hemisphere.

"These latest observations are very exciting, they confirm the existence of an atmosphere which we predicted from the distant earlier flybys and they will also allow us to gain a much better understanding of the processes taking place which are producing this very exotic atmosphere" - Professor Michele Dougherty, Imperial College London, Principal Investigator on the Magnetometer instrument on Cassini.

The tortured southern polar terrain of Saturn's moon Enceladus appears strewn with great boulders of ice in these two fantastic views - the highest resolution images obtained so far by Cassini of any world. This comparison view consists of a wide-angle camera image (left) for context, and a high-resolution narrow-angle camera image (right). The two images were acquired at an altitude of approximately 208 kilometres, as Cassini made its closest approach yet to Enceladus.

The wide-angle view shows what appears to be a geologically youthful, tectonically fractured terrain. In the narrow-angle view, some smearing of the image due to spacecraft motion is apparent. Both of these views were acquired as Enceladus raced past Cassini's field of view near the time of closest approach. At the time, the imaging cameras were pointed close to the moon's limb (edge), rather than directly below the spacecraft. This allowed for less 'motion blur' than would have been apparent had the cameras pointed straight down. Thus, the terrain imaged here was actually at a distance of 319 kilometres from Cassini.

At the fine scale afforded by the Cassini spacecraft narrow-angle view, the surface is dominated by ice blocks between 10 and 100 meters across. The origin of these icy boulders is enigmatic. Scientists are interested in studying the sizes and numbers of the blocks in this bizarre scene, and in understanding whether terrain covered with boulders is common on Enceladus.

The image scale is about 4 metres per pixel in the narrow-angle image and about 37 metres per pixel in the wide-angle image. The wide-angle image has been magnified by a factor of two. The contrast in both images has been enhanced to improve the visibility of surface features.

Cassini spacecraft has obtained new detailed images of the south polar region of Saturn's moon Enceladus. The data reveal distinctive geological features and the most youthful terrain seen on the moon. These findings point to a very complex evolutionary history for Saturn's brightest, whitest satellite.

Cassini's July 14 flyby brought it within 175 km of the surface of the icy moon. The close encounter revealed a landscape near the south pole almost entirely free of impact craters. The area is also littered with house-sized ice boulders carved by tectonic patterns unique to Enceladus. These features set the region apart from the rest of the moon.

As white as fresh snow, Enceladus has the most reflective surface in the solar system. Previous Cassini flybys revealed Enceladus, in contrast to Saturn's other icy moons, has lightly cratered regions, fractured plains and wrinkled terrain. The new findings add to the story of a body that has undergone multiple episodes of geologic activity spanning a considerable portion of its lifetime. The moon's southern most latitudes have likely seen the most recent activity.

These same latitudes may also bear the scars of a shift in the moon's spin rate. If true, this speculation may help scientists understand why Enceladus has a tortured-looking surface, with pervasive crisscrossing faults, folds and ridges. The most remarkable images show ice blocks, about 10 to 100 metres across in a region that is unusual in its lack of the very fine-grained frost that seems to cover the rest of Enceladus.

"A landscape littered with building-sized blocks was not expected. The minimal cover of finer material and the preservation of small, crossing fracture patterns in the surrounding areas indicate that this region is young compared to the rest of Enceladus" - Dr. Peter Thomas, imaging-team member from Cornell University, Ithaca, N.Y.

False colour composites of this region, created from the most recent images, show the largest exposures of coarse-grained ice fractures seen anywhere on the satellite, which also supports the notion of a young surface at southern latitudes. Some of the latest images may hint at the answer. The images revealed additional examples of a distinctive "Y-shaped" tectonic feature on Enceladus. In this unusual element, parallel ridges and valleys appear to systematically fold and deform around the south polar terrains.

"These tectonic features define a boundary that isolates the young, south polar terrains from older terrains on Enceladus. Their placement and orientation may tell us a very interesting story about the way the rotation of Enceladus has evolved over time and what might have provided the energy to power the geologic activity that has wracked this moon" - Dr. Paul Helfenstein, associate of the imaging team also at Cornell University.

The apparent absence of sizable impact craters also suggests the south pole is younger than other terrain on Enceladus. These indications of youth are of great interest to scientists who have long suspected Enceladus as one possible source of material for Saturn's extensive and diffuse E ring, which coincides with the moon's orbit. Young terrain requires a means to generate the heat needed to modify the surface. Other Cassini instrument teams are working to understand data about temperature, composition, particles and magnetic field. Together with image interpretation, these data can create a more complete picture.

As it swooped past the south pole of Saturn's moon Enceladus on July 14, 2005, Cassini acquired high resolution views of this puzzling ice world. From afar, Enceladus exhibits a bizarre mixture of softened craters and complex, fractured terrains. This large mosaic of 21 narrow-angle camera images have been arranged to provide a full-disk view of the anti-Saturn hemisphere on Enceladus. This mosaic is a false-colour view that includes images taken at wavelengths from the ultraviolet to the infrared portion of the spectrum, and is similar to another, lower resolution false-colour view obtained during the flyby. In false-colour, many long fractures on Enceladus exhibit a pronounced difference in colour (represented here in blue) from the surrounding terrain.

A leading explanation for the difference in colour is that the walls of the fractures expose outcrops of coarse-grained ice that are free of the powdery surface materials that mantle flat-lying surfaces. The original images in the false-colour mosaic range in resolution from 350 to 67 meters per pixel and were taken at distances ranging from 61,300 to 11,100 kilometres from Enceladus. The mosaic is also part of a movie sequence of images from this flyby.

On 14 July, Cassini swooped in for an unprecedented close-up view of the wrinkled moon. Its Imaging Science Subsystem (ISS) camera has since returned pictures of a boulder-strewn landscape that is currently beyond explanation. The "boulders" appear to range between 10 and 20 metres in diameter in the highest-resolution images, which can resolve features just 4 m across.

“That’s a surface texture I have never seen anywhere else in the solar system,” says David Rothery, a planetary geologist at the Open University in Milton Keynes, UK.

Cracks crisscross Enceladus's surface - possibly as a result of the moon being repeatedly squeezed and stretched by the gravity of Saturn and other moons nearby. But Rothery points out the boulders avoid - rather than fill - the cracks. This might indicate that the fracturing took place after the boulders had already formed.Alien landscape

John Spencer, a Cassini team member at the Southwest Research Institute in Boulder, Colorado, US, agrees that the images are puzzling. “You would expect to see small craters or a smooth, snow-covered landscape at this resolution," he told New Scientist. "This is just strange. In fact, I have a really hard time understanding what I’m seeing.”

NASA scientists have been locked in discussions since 15 July and are expected to pass judgment on what they think this peculiar surface might be later on Tuesday.

But Elizabeth Turtle, a Cassini imaging team member at the University of Arizona in Tucson, US, warns there will be no quick answers. “Trying to figure out what is going on is going to take a lot longer than a weekend of swapped emails,” she says.

Expand This image was taken with the narrow angle camera from a distance of approximately 68,320 kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 50 degrees. Resolution in the image is about 410 metres per pixel.

Expand This image was taken on July 14, 2005 and received on Earth July 15, 2005. The camera was approximately 35,276 kilometres away from the surface.The image was taken using the CL1 and CL2 filters.

Expand This image was taken with the narrow angle camera from a distance of approximately 24,380 kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 50 degrees. Resolution in the image is about 290 metres per pixel.

Expand This image was taken with the narrow angle camera from a distance of approximately 33,610 kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 50 degrees. Resolution in the image is about 200 metres per pixel.